In a telephone set, cost considerations dictate the quality and size of speakers used. Typical 50 mm to 66 mm diameter telephone speakers have a first resonance frequency from about 220 to 350 Hz whereas smaller speakers can have a first resonant frequency close to 500 Hz. When the speakers are enclosed in a sealed box, these frequencies shift up consistently, such that it can be difficult to meet specifications for wide band telephony hands free operation (150 Hz-7 kHz).
The conventional method for increasing low frequency response is to use a bass-reflex design. Small speaker enclosures give rise to the problem of vortex noise for which a number of solutions have been proposed, such as those disclosed by Van Schyndel in U.S. Pat. No. 5,757,946, Roozen in U.S. Pat. No. 6,275,597, and Polk in U.S. Pat. No. 5,809,154. These solutions all teach the use of a single enclosure to achieve a desired frequency response.
Stereo loudspeaker enclosures of various types are omnipresent in audio-visual products either as separate components (e.g. sound systems, personal computers) or embedded within products (e.g. televisions, computer monitors, etc.) In all such cases the two enclosures (left and right) are of identical acoustical response and usually of the same size and shape. In some cases, stereo speakers have the low frequency augmented by the use of a third loudspeaker (sub-woofer) to cover the very low frequency range where humans have difficulty localising the source.
Sound reinforcement systems represents another application in which multiple enclosures are used. Specifically, different enclosures of different dimensions may be used for different frequency ranges (e.g. a two-way system comprising a high frequency cabinet and a low frequency cabinet). In such systems the various cabinets are usually clustered together.
In all of the foregoing systems it is desirable that speaker coverage of a specific frequency band be accomplished with as flat a frequency response as possible. Where multiple drivers or enclosures are used, the overlap in frequency response is minimised and is designed so that the transition from one range to another is as smooth as possible.